Solar cells have been manufactured using a light absorption layer formed at high cost and silicon (Si) as a semiconductor material since an early stage of development. To more economically manufacture commercially viable solar cells, structures of thin film solar cells, using an inexpensive light absorbing material such as copper indium gallium sulfo (di)selenide (CIGS) or Cu(In, Ga)(S, Se)2, have been developed. Such CIGS-based solar cells typically include a rear electrode layer, an n-type junction part, and a p-type light absorption layer. Solar cells including such CIGS layers have a power conversion efficiency of greater than 19%. However, in spite of potential for CIGS-based thin film solar cells, costs and insufficient supply of In are main obstacles to widespread commercial application of thin film solar cells using CIGS-based light absorption layers. Thus, there is an urgent need to develop solar cells using In-free or low-cost universal elements.
Accordingly, as an alternative to the CIGS-based light absorption layer, CZTS(Cu2ZnSn(S,Se)4)-based solar cells including copper (Cu), zinc (Zn), tin (Sn), sulfur (S), or selenium (Se), which are extremely cheap elements, have recently received attention. CZTS has a direct band gap of about 1.0 eV to about 1.5 eV and an absorption coefficient of 104 cm−1 or more, reserves thereof are relatively high, and CZTS uses Sn and Zn, which are inexpensive.
In 1996, CZTS hetero junction PV batteries were reported for the first time, but CZTS-based solar cells have been advanced less than CIGS-based solar cells and photoelectric efficiency of CZTS-based solar cells is 10% or less, much lower than that of CIGS-based solar cells. Thin films of CZTS are manufactured by sputtering, hybrid sputtering, pulsed laser deposition, spray pyrolysis, electro-deposition/thermal sulfurization, e-beam processing, Cu/Zn/Sn/thermal sulfurization, and a sol-gel method.
With regard to fabrication methods, WO2007-134843 discloses a method of forming a CZTS layer by simultaneously or sequentially stacking Cu, Zn, and Sn via vacuum sputtering and heat-treating the resulting material under an S or Se atmosphere. Some papers ((Phys, Stat. Sol. C. 2006, 3, 2844. /Prog. Photovolt: Res. Appl. 2011; 19:93-96) disclose a method of forming a CZTS layer by simultaneously depositing Cu, Zn, Sn, S, or Se on a base by simultaneous vacuum evaporation. However, the above-described related art is advantageous in that deposition is performed in a relatively well-controlled state, but is disadvantageous in that manufacturing costs are high due to use of expensive equipment.
Meanwhile, PCT/US/2010-035792 discloses formation of a thin film through heat treatment of ink including CZTS/Se nanoparticles on a base. With regard to methods of forming CZTS precursor particles, Journal, J. Am. Chem. Soc., 2009, 131, 11672 discloses that CZTS nanoparticles are formed by mixing a solution including Cu, Sn and Zn precursors and a solution including S or Se at high temperature through hot injection. In addition, US2011-0097496 discloses a method of forming a CZTS layer using a precursor for forming the CZTS layer, prepared by dissolving Cu, Zn, Sn salts together with an excess of S or Se in hydrazine, through heat treatment and selenization in subsequent processes. However, hot injection has a problem regarding stability. In addition, hydrazine including a chalcogen compound containing an excess of S or Se is a highly toxic, highly reactive, highly explosive solvent and thus a solution process using hydrazine entails high risk. Furthermore, hydrazine is difficult to handle and thus there are difficulties in manufacturing processes.
Therefore, there is a high need to develop a technology for thin film solar cells including a high efficiency light absorption layer and being stable against oxidation using a fabrication method that is less expensive and safer than an existing CZTS layer preparation method.